Cation exchange materials are finding increased acceptance in electrolytic cells, and particularly in chlorine generating cells. The cation exchange membranes in such cells are utilized to divide the cells into anode and cathode compartments. These membranes function to pass metal cations arising from electrolytic activity at the cell anode from the anode to the cathode compartment. At the same time, these membranes substantially resist movement of anions from the cell cathode to the anode compartment. The membranes assure a cathode product substantially free of contamination by electrolytes present in the anode compartment of the cell while reducing electrical inefficiency in cell operation by reducing migration of anions from the cathode to the anode compartment where such anions would react.
While a variety of cationic exchange membranes have been utilized in fabricating electrolytic cells, one material in particular has found acceptance. Membranes fabricated from copolymeric vinyl ethers, products of E. I. duPont called Nafion.RTM. have produced superior results in electrochemical cells and particularly in chlorine cells.
These NAFION compounds are known as perfluorocarbons and are a copolymer of at least two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.
The second monomer often is selected from a group of monomers usually containing an SO.sub.2 F or sulfonyl fluoride group. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising generally 1 to 8 carbon atoms but upon occasion as many as 25. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbon atom adjacent the --SO.sub.2 F group, particularly where the functional group exists as the --(--SO.sub.2 NH)mQ form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups.
Chlorine cells equipped with separators fabricated from perfluorocarbon copolymers have been utilized to produce a somewhat concentrated caustic product containing quite low residual salt levels. Perfluorocarbon copolymers containing perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer have found particular acceptance in Cl.sub.2 cells.
Ion exchange functionality is generally imparted to such ion exchange membranes by saponification with caustic or the like. In chlorine cells using a sodium chloride brine feedstock, one drawback to the use of perfluorocarbon separators having pendant saponified sulfonyl fluoride based functional groups has been a relatively low resistance in desirably thin separators to back migration of caustic including OH.sup.- radicals from the cathode to the anode compartment. This back migration contributes to a lower current utilization efficiency in operating the cell since the OH.sup.- radicals react at the anode to produce oxygen. It is known that amination of these groups can produce enhanced back migration resistance but also interferes with cation migration requiring higher cell voltages. Recently, it has been found that if pendant sulfonyl fluoride based cationic exchange groups adjacent one separator surface were converted to pendant carboxylate groups, the back migration of OH.sup.- radicals in such Cl.sub.2 cells would be significantly reduced. Conversion of sulfonyl fluoride groups to carboxylate groups is discussed in U.S. Pat. No. 4,151,053.
Presently, perfluorocarbon separators are generally fabricated by forming a thin membrane-like sheet under heat and pressure from one of the intermediate copolymers previously described. The ionic exchange capability of the copolymeric membrane is then activated by saponification with a suitable or conventional compound such as a strong caustic. Generally, such membranes are between 0.5 mil and 150 mil in thickness. Reinforced perfluorocarbon membranes have been fabricated, for example, as shown in U.S. Pat. No. 3,925,135.
Presently, saponified or aminated copolymer or perfluorocarbon copolymer resin structures having sulfonyl based functionality and becoming useless for whatever reason are generally discarded. Were it convenient to convert the functional groups upon these resin structures back to SO.sub.2 F, these resins could then be recycled, resulting in significant cost savings. Further, where it is desired that surface portions of a saponified or aminated NAFION having sulfonyl fluoride based functional groups be converted to an alternate functionality, this conversion is often more readily accomplished from SO.sub.2 F functional groups.